[Tutor] Decrypting a Password

[Peter Otten]

Linda Gray wrote:

> Hello,
> 
> I am working on a homework assignment that has me creating a password
> saver
> using a ceasar cipher code.  I was provided the key to the cipher and two
> passwords.  I need to look up and decrypt the passwords and create a
> program to add a password and delete a password (options 2, 3 and 7).  I
> had no problems adding a password.  I am also having no problems looking
> up a password but am havving problems decrypting the password. 

The good news is that when you have a function that correctly encrypts a 
string using caesar's cipher you also have one that decrypts it -- because 
its the same function. Let's try:

>>> def passwordEncrypt (unencryptedMessage, key):
...     #We will start with an empty string as our encryptedMessage
...     encryptedMessage = ''
...     #For each symbol in the unencryptedMessage we will add an encrypted 
symbol into the encryptedMessage
...     for symbol in unencryptedMessage:
...         if symbol.isalpha():
...             num = ord(symbol)
...             num += key
...             if symbol.isupper():
...                 if num > ord('Z'):
...                     num -= 26
...                 elif num < ord('A'):
...                     num += 26
...             elif symbol.islower():
...                 if num > ord('z'):
...                     num -= 26
...                 elif num < ord('a'):
...                     num += 26
...             encryptedMessage += chr(num)
...         else:
...             encryptedMessage += symbol
...     return encryptedMessage
... 
>>> encrypted = passwordEncrypt("Hello world!", 16)
>>> encrypted
'Xubbe mehbt!'
>>> passwordEncrypt(encrypted, -16)
'Hello world!'

That's your function (without the empty lines to allow pasting into the 
interactive interpreter without syntax errors), and it seems to work.


> I can only
> get it to provide me the give, encrypted password and not the unencrypted
> one

The line
>                      print(passwordunEncrypt(passwords[i][1], 16))

is incorrectly indented making it part of the passwordunEncrypt() function. 
Therefore it is never executed.

It's generally a good idea to put functions at the beginning of a script and 
to define them on the module level rather than at some arbitrary point in a 
control structure (even experts hardly ever do that).

> which is the one I need to provide.  I am not getting any errors, it is
> just not doing anything that I can see.  I am getting the following error
> with the deleting a password.
> 
> Traceback (most recent call last):
>   File "C:/Users/lrgli/Desktop/Python Programs/Password test file.py",
>   line
> 187, in <module>
>     passwords.remove (passwordToDelete)
> ValueError: list.remove(x): x not in list
> 
> Any assistance, guidance, pointers would be appreciated.  Is my
> indentation wrong for the decryption, am I missing something connecting
> the two, etc.?

The list contains lists (the site/password pairs) and remove looks for the 
password alone. A small example:

>>> items = [["foo", "bar"], ["ham", "spam"]]
>>> items.remove("foo")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: list.remove(x): x not in list

For the removal to succeed you need to provide the exact entry:

>>> items.remove(["foo", "bar"])
>>> items
[['ham', 'spam']]

A workaround would be to loop over the items

>>> items = [["foo", "bar"], ["ham", "spam"]]
>>> for i, item in enumerate(items):
...     if item[0] == "foo":
...         del items[i]
...         break # required!
... 
>>> items
[['ham', 'spam']]

but Python has an alternative that is better suited for the problem: the 
dictionary:

>>> lookup = {"foo": "bar", "ham": "spam"}
>>> del lookup["foo"]
>>> lookup
{'ham': 'spam'}


> I feel like I am close.  It is python version 3.
> 
> Here is my code: